1. Field of the Invention
The present invention relates to a radiation image detector for recording radiation images by generating charges by irradiation with radiation and by storing the charges.
2. Description of the Related Art
Conventionally, in the medical field or the like, various kinds of radiation image detectors that record radiation images related to a subject have been proposed and used in practical situations. The radiation image detectors record the radiation images related to the subject by irradiation with radiation that has passed through the subject and output electric signals based on the recorded radiation images.
The radiation image detectors are classified into a direct-conversion type and an indirect-conversion type. In the direct-conversion type, radiation is directly converted into charges and the charges are stored (accumulated). In contrast, in the indirect-conversion type, radiation is temporarily converted into light by using a scintillator, such as CsI:Tl and GOS (Gd2O2S:Tb), and the light is converted into charges in the photoconductive layer. Then, the charges are stored. Further, the readout types of the radiation image detectors are basically classified into a light readout type using readout light and an electrical readout type using a thin film transistor (TFT).
As the light-readout-type radiation image detector, the following radiation image detector is known, for example. In the light-readout-type radiation image detector, charges generated in a photoconductive layer for recording by irradiation with radiation are stored and linear electrodes are charged (electrified) with charges that have the opposite polarity to the polarity of the stored charges. Pairs of charges (charge pairs, dipoles or the like) are generated in the photoconductive layer for readout by irradiation with readout light and the charges of the pairs of charges that have two different polarities are combined with the stored charges and the charges that have charged the linear electrodes, respectively. Accordingly, the stored charges are read out.
As the electrical-readout-type radiation image detector, the following radiation image detector is known, for example. In the electrical-readout-type radiation image detector, charges generated by irradiation with radiation are collected by pixel electrodes for respective pixels and stored in a storage capacity (accumulation capacity) connected to the pixel electrodes. Then, the stored charges are read out, pixel by pixel, by turning on/off electrical switches, such as thin film transistors (TFT's), a charge coupled device (CCD), or a complementary metal oxide semiconductor (CMOS) sensor.
A technique of covering the entire surfaces of pixel electrodes with an insulation coating or a coating made of an insulating substance containing carbon particles or metal particles in an electrical-readout-type radiation image detector is disclosed in Japanese Unexamined Patent Publication No. 2006-156555. In the technique, the pixel electrodes are covered in such a manner to make the surface of the coating smooth (even) and to improve the properties of the coating.
Further, U.S. Pat. No. 5,861,052 discloses a technique of providing a semiconductor that separately covers each of edge portions of the pixel electrodes in an electrical-readout-type radiation image detector. The semiconductor is provided to prevent charges that have once trapped by the pixel electrodes from moving to other pixels electrodes adjacent thereto.
The aforementioned linear electrodes and pixel electrodes are detection electrodes for detecting signals based on charges generated by irradiation with radiation. In such detection electrodes, an electric field tends to concentrate in the edge portions thereof. Therefore, charges tend to be injected into the photoconductive layer from the edge portions of the detection electrodes. As the material for the photoconductive layer, an amorphous (non-crystalline) substance, such as amorphous selenium, is generally used. If excessive charges are injected into the photoconductive layer from the edge portions of the detection electrodes, heat is generated at the portion of the photoconductive layer into which the charges have been injected. Consequently, crystallization occurs in the photoconductive layer. If crystallization occurs in the photoconductive layer, an artifact is generated in a detected image because the electrical property of the crystallized portion and that of the amorphous portion differ from each other. Consequently, the image quality deteriorates.
Such injection of charges can be effectively prevented by covering the pixel electrodes with an insulating substance. Japanese Unexamined Patent Publication No. 2006-156555 discloses structure in which pixel electrodes are covered with an insulation coating. However, the entire surfaces of the pixel electrodes that include not only the edge portions of the pixel electrodes but flat portions of the pixel electrodes are covered. Therefore, the charge transfer characteristic becomes lower and the sensitivity and the residual image characteristic (afterimage characteristic) deteriorate. Japanese Unexamined Patent Publication No. 2006-156555 discloses a technique of covering the entire surfaces of the pixel electrodes with a coating containing particles that have a charge transfer characteristic to improve electrical conductivity. However, in this structure, charges tend to be injected into the photoconductive layer Consequently, crystallization of the photoconductive layer tends to occur.
U.S. Pat. No. 5,861,052 discloses structure in which the edge portions of the pixel electrodes are covered with a semiconductor. The electrical conductivity of the semiconductor is high. Therefore, the sensitivity and the residual image characteristic do not deteriorate, but the amount of charges injected into the photoconductive layer increases. Therefore, the structure is not effective to prevent crystallization of the photoconductive layer.
Further, even if injection of excessive charges into the photoconductive layer is reduced, there is a problem that crystallization of the photoconductive layer occurs and progresses by using the radiation image detector repeatedly.